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In the 3rd century BCE, Eratosthenes calculated the circumference of the Earth to high precision.
Further Reading:
http://en.wikipedia.org/wiki/Eratosthenes
This is a short segment from the series Cosmos by the late Carl Sagan. The most poetic popularizer of science in the 20th century.
If you have never seen Cosmos, I strongly recommend it. It's full of great stories like this one. It is available on DVD, in 7 subtitle languages. Purchase links are below.
http://www.carlsagan.com/
http://sh